1. Field of the Invention
The present invention relates generally to implantable cardiac stimulation devices and systems for regulating the contraction of a heart. More particularity, the invention relates to a defibrillation lead, and more particularly to a defibrillation lead having a distal electrode and a multi-lumen lead body.
2. Description of the Related Art
Implantable medical devices for treating irregular contractions of the heart with electrical stimuli are well known in the art. Some of the most common forms of such implantable devices are defibrillators and pacemakers.
Defibrillators are implantable medical devices used to treat fibrillation, a condition characterized by rapid, chaotic electrical and mechanical activity of the heart's excitable myocardial tissue that results in an instantaneous cessation of blood flow from the heart. Defibrillation is a technique employed to terminate fibrillation by applying one or more high energy electrical pulses to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to restore the normal synchronized contraction of the total mass of tissue.
A pacemaker, or pacer, is an implantable medical device that delivers low energy electrical pulses to stimulate a patient's heart to beat at a desired rate in instances where the heart itself is incapable of proper self-regulation. This occurs when the heart's natural pacemaker, which causes the rhythmic electrical excitation of the heart and pumping of blood, malfunctions due to age or disease. Pacing is a process used to maintain normal beating of a heart having this condition.
Various types of leads for defibrillators and pacers have been suggested in the prior art. For example, large electrical patches sewn to the exterior surface of the heart have been used to deliver defibrillation pulses to the heart. Implantation of such patch electrodes requires opening of the patient's chest during thoracic surgery. For pacing, pulses may be applied to the heart with the use of a pacer lead having an exposed metal surface, or pacer electrode, extending through a vein and into the heart.
Those involved in the medical arts recognized that prior art defibrillators required a high threshold level of energy for effective defibrillation, which limited the useful life-span of the devices and, more significantly, posed a significant risk of causing electrolysis of the blood and myocardial damage. It was realized that the defibrillation electrode configuration played an important role in the amount of energy needed to achieve successful defibrillation. This led to the development of transvenous defibrillation leads having long coil-shaped defibrillation electrodes for implantation into the right ventricle of the heart through a vein. For example, U.S. Pat. No. 4,922,927, the entire disclosure of which is incorporated herein by reference, discloses a defibrillation electrode made up of a plurality of separate wires wound side-by-side to form a tight coil. The coil was disposed upon an insulated tubular member and had a length sufficient to extend throughout the entire length of the ventricular chamber to provide sufficient electrode surface area for defibrillation.
Transvenous cardiac stimulation leads, such as the device of U.S. Pat. No. 4,922,927, were configured to also carry a pacing electrode. Thus, a single device implantable in one surgical procedure could provide defibrillation and pacing pulses for heart patients suffering from both irregular heart beat and, at times, cardiac fibrillation. This eliminated the need for multiple and complex surgical procedures to attach the prior art electrodes required for both types of treatments.
Another defibrillation electrode configuration for use with dual purpose transvenous leads is disclosed in U.S. Pat. Nos. 5,476,502 and 5,374,287 to Rubin, which are also incorporated herein by reference in their entireties. The "Rubin" catheter included either a helical or lance shaped defibrillation electrode for delivering a defibrillation pulse directly to the interior of the septum of the patient's heart. The length of the helix-shaped electrode to be screwed into the septum from the right ventricle, about 0.5 cm to 1.0 cm, was substantially shorter than the conventional coiled transvenous defibrillation electrodes.
Implantable cardiac leads typically have a distal electrode at the extreme distal end of the lead. In cardiac pacemaker leads this electrode is usually a cathodic electrode conducting low voltage pulses which stimulate the heart to contract. Modern endocardial defibrillation leads are now used with devices which provide cardiac pacing as well as defibrillation shock therapy. In such leads, a distal pacing electrode is usually provided, also.
Endocardial defibrillation leads were constructed using co-axial construction typical of pacemaker leads. That is, the lead comprised an outer insulating lead body of silicon rubber or polyurethane with a central lumen. One or more coiled conductors extend through the lumen. These conductors were placed one inside the other and had a common axis with the axis of the lumen in the lead body. The conductors were separated by insulating tubes. The entire assembly was symmetrical around a common axis. Consequently, it was easy to insert an axially symmetrical cathodic electrode at the distal tip.
More recently, defibrillation leads have used lead bodies with multiple lumens, providing additional insulating separation between low voltage and high voltage conductors. Because such lead bodies do not have a lumen symmetrical about the axis of the lead body, it has been difficult to attach the distal electrode, which is typically symmetrical about the lead axis. The most common solution has been to glue a short, tapered tube to the distal end of the lead body. The distal electrode could then be inserted into this tube. This technique expands the diameter of the lead near the distal tip. In addition, It introduces an additional glue joint.
There continues to be a need for improved multi-lumen cardiac leads with distal electrodes which are axially mounted.